Interpretive Summary: Sunlit growth chambers are a very useful tool to study environmental impacts on plant growth and development such as rising temperatures, elevated carbon dioxide, and ozone in the air. An important feature of the sunlit growth chambers is that plants are grown under natural light unlike artificially-lit indoor growth chambers, while other environmental factors can still be controlled precisely. It is important to understand and characterize how the light environment inside those chambers compares with outside. This paper demonstrates that visible light environment inside the sunlit chambers including our Soil-Plant-Atmosphere-Research (SPAR) units are fairly close to the ambient light in quantity. It also illustrates that there could be variations in light availability inside the sunlit chambers due to reflections and variable transmissions by the chamber walls. A mathematical model was developed to identify the light level at a given location inside a sunlit chamber. This paper can benefit researchers who conduct experiments using the sunlit chambers to better understand the light environment inside their chambers.

Technical Abstract:
Naturally sunlit, outdoor growth chambers allow plants to grow under natural light while controlling other environmental variables. Variable transmissions and reflections by chamber walls could attenuate photosynthetically active radiation (PAR) within sunlit chambers from the ambient levels depending on sun angles and with locations inside the chamber. It is important to understand and characterize actual PAR environment inside the sunlit chambers. This study identified diurnal patterns and distribution of PAR inside various types of sunlit growth chambers including Soil-Plant-Atmosphere-Research (SPAR) units using single-point and line quantum sensors. On a clear day, the diurnal patterns of PAR were different from ambient PAR at some locations within the SPAR unit, reaching higher PAR than outside during the time of the day where two reflections from adjacent walls overlap. Daily integrals of PAR measured using line quantum sensors placed within crop growing area were 93% and 105% of the ambient PAR on a clear day inside a SPAR unit. A gradient in PAR within the chambers existed from south to north due to the reflections by the northern interior chamber wall. This gradient was less pronounced on a cloudy day and daily PAR within the crop growing area was distributed between 92% and 95% of the ambient PAR. A mathematical model was developed to simulate incident PAR inside sunlit chambers. The model accounts for solar geometry, direct and diffuse radiations, chamber geometry, and chamber optical properties. The model was capable of simulating the diurnal patterns of PAR inside various types of sunlit chambers. Model solutions indicated that total daily PAR inside sunlit chambers was usually within 5% of ambient PAR. Spatial distribution of daily PAR ranged from 75% to 120% inside various types of sunlit chambers including SPAR and open-top chambers.